Connections to Chemistry Concepts (for correlation to course curriculum)
Organic chemistry—The ability of carbon to form multiple bonds—with itself and with other elements—is the reason for the existence of polymers.
Petroleum chemistry—It would be good for students to understand how versatile a mixture crude oil is as a feedstock for many industries, and that mankind shouldn’t be using it solely as a fuel in cars (gasoline) of furnaces (fuel oil).
Polymers— Although polymers don’t usually show up in a typical chemistry curriculum until near the end of the year, if at all, the ubiquity of polymers in our lives makes this a topic of high interest for students.
Secondary (intermolecular) bonding—While individual secondary bonds are not very strong (think dipole-dipole interactions or van der Waals forces), the huge number of intermolecular bonds involved in macromolecular polymer molecules accounts for many of the properties of polymers.
Reversibility of reactions—The chemical method of recycling shows that some types of polymerization reactions can be reversed. Information in the “More on chemical recycling” can be used to discuss the feasibility of doing this type of reaction, in terms of energy consumption. The reversibility of these reactions could lead to a discussion of equilibrium.
Sustainability—Recycling is a major way we can avoid using up all our natural resources
Industrial chemistry—Students don’t often get a chance in high school chemistry to see chemistry related to industrial plants. This would be a good time to discuss chemistry’s central role in manufacturing all the goods students use/consume every day.
Phase changes—Thermoplastic molding is a phase change from liquid (heated) to solid (cooled). It involves only intermolecular bonding, so it is a physical change. Thermosets also involve a phase change from liquid (heated) to solid (cooled), but this is both a physical change and a chemical change, because primary chemical bonds—crosslinks—form between the polymer strands, forming a much larger polymer molecule.
Physical changes vs. chemical changes—see 8 above. Physical changes are easily reversible—thermoplastics can be heated and reform the solid; chemical changes are not easily reversible—thermosets cannot be heated and reformed. Also, physical vs. chemical recycling also illustrates these differences between physical and chemical changes.
Safety—The hazards of recycling might provide a good opportunity to discuss safety in both industry and lab settings.
Possible Student Misconceptions (to aid teacher in addressing misconceptions)
“All plastic is the same, right?” Actually, students should already know this isn’t true, if they take a moment to look closely at some of the plastics around them. Transparent plastic wrap looks nothing like a translucent milk jug, for instance, even though they are similar chemically. A flexible, transparent water bottle doesn’t have any of the properties of a piece of rigid, opaque PVC (polyvinyl chloride) pipe. Each type of plastic has its own set of unique properties that make it better for some purposes than for others.
“When the guys take my plastic stuff for recycling, they just throw the whole pile together and melt it down to make new stuff.”As the article mentions, in order to recycle, the first thing the processing plant has to do is to sort the plastic material into different piles based on the recycle code of the plastic. A mix of plastics melted down would result in a plastic that is of very little use because it would have a mix of properties of the individual plastics—and none of their strengths.
“Polymers are all plastics; plastics are the only polymers.”Plastics are only a small segment of the polymer “population”. In synthetic polymers, there are also films (e.g., plastic wraps) and coatings (e.g., paints), fibers (e.g., nylon, Spandex, etc.) adhesives (e.g., superglue and hair spray) and elastomers (e.g., tires and balloons). Natural polymers comprise almost all living matter; e.g., DNA, proteins, cells, tissue, organs, etc. There are also natural films and coatings (e.g., “milk” from milkweed), fibers (e.g., cotton and wool), adhesives (e.g., barnacle secretions), and elastomers (e.g., natural rubber from rubber trees).
“All plastics ‘live forever’—there aren’t any that degrade quickly or easily.”While most plastics do have long lives, some plastics are purposely manufactured to degrade quickly, either by action of bacteria (biodegradable, or light (photodegradable). For example, six-pack rings from cans of carbonated beverages were required to be made photodegradable by ultraviolet light (from the sun) in Federal law 40CFR238.30, in order to protect fish, birds and other wildlife. (http://edocket.access.gpo.gov/cfr_2003/julqtr/40cfr238.30.htm)
Other plastics have been made to be biodegradable, upon exposure to moisture, air and bacteria. The problem with this is that most plastic is buried in landfills, where air is not abundantly available, so biodegradation is still a slow process in these conditions.
“We made a polymer in class today—we made ‘slime’.”“Slime” is indeed a polymer, but you didn’t make it a polymer—it already was a polymer. “Slime” is made of polyvinyl alcohol and borax, or Elmer’s glue and borax; however, the polymer was already there in the original reactants—polyvinyl alcohol is a polymer and Elmer’s glue contains polyvinyl acetate; again, already a polymer. The addition of borax solution to either of these polymers merely cross-links the polymer chains to make the whole entity less fluid and not as flexible, to make into “slime”.
“So, if we recycle all our plastics, we’ll never run out of raw material to make new stuff.”That would be really great. Unfortunately, each time we recycle a plastic item, the plastic degrades a bit and loses some of its desirable properties. That’s why companies using recycled plastic will typically add some virgin resin (plastic) to the recycled material, to help maintain the original properties. There is a limit to how often plastic can be recycled.